The present invention relates generally to Doppler blood flow measurement systems and techniques, and more particularly to Doppler blood flow measurement systems and techniques using the frequency domain signal analysis.
During cardiopulmonary bypass surgery, ventricular assist using blood pumps and other cardiac surgeries, blood flow external to the patient is necessary. Known blood pumps and so-called heart-lung machines operate to transport the blood of the patient through tubing or conduits in order to perform their function. During the transportation of blood in these external (to the body of the patient) tubes or conduits, it is extremely important for the surgeon to monitor the rate of flow of the blood so that abnormalities in the flow can be detected and corrective action can be taken.
Various systems and techniques have been utilized to measure the flow of blood, or other fluids, through tubes or conduits in the past.
Invasive measurement systems including techniques such as vane type flow meters not only require either disposal or sterilization after each use, but, with blood, may lead to unwanted coagulation or other problems. U.S. patent application Ser. No. 07/074,549, Lloyd C. Hubbard and Earl W. Clausen, filed July 17, 1987, entitled FLOW MEASUREMENT SYSTEM, assigned to Minnesota Mining and Manufacturing Company who is the assignee of the present invention, describes a blood flow measurement system for use with a motor driven centrifugal pump. The system takes advantage of the fact that, at a constant speed of rotation and a constant viscosity, the torque required to drive a centrifugal pump is directly related to the flow produced by the pump. Blood flow is computed from the speed of rotation of the pump and the torque of the motor.
The use of ultrasound to determine the flow of blood in a blood vessel started generally in the 1950's. Some of these ultrasound systems were implanted into the patient and some utilized measurements taken external to the patient.
The ultrasonic measurement of blood flow through tubes or conduits using the known Doppler frequency shift effect has been utilized. Such a measurement system and technique has the distinct advantage of being non-invasive. The tube or conduit, being relatively transparent to the ultrasonic waves, need not be physically invaded. In such known systems and techniques an ultrasonic transmitter is placed angularly with respect to the expected blood flow through the tube or conduit. An ultrasonic receiver is angularly placed on the opposite or same side of the tube or conduit. The presence of particulates, such as red blood cells, air bubbles and fat globules, act as targets for the reflection of the ultrasonic signal. The velocity of these targets cause a frequency shift in the reflected ultrasonic frequency according to the well known Doppler effect.
An example is a prior flowmeter marketed by Sarns, Inc. of Ann Arbor, Mich. (now a subsidiary of Minnesota Mining and Manufacturing Company, St. Paul, Minn., the assignee of the present application) known as the Sarns model 7800 flowmeter. An accuracy of about .+-. ten percent (10%) was achievable with this device. Indeed, in order to achieve this accuracy the console of each flowmeter must be matched to an individual flowprobe at the time of manufacture. Due to the matching requirement, manufacturing and field service was made more difficult and interchangeability of probes between flowmeters could not be achieved.
The system described in U.S. Pat. No. 4,690,002, Hubbard et al, also assigned to Minnesota Mining and Manufacturing Company, is an example of an ultrasonic Doppler blood flow measurement system. This system operates on an analog basis by amplifying the reflected signal, clipping it, using automatic gain control to restrain the signal into a reasonably finite range and converting the signal from a frequency to a voltage by use of an analog frequency-to-voltage converter.
In Atkinson, Peter, "A Fundamental Interpretation of Ultrasonic Doppler Velocimeters", Ultrasound in Medicine & Biology, Volume 2, pp. 107-111, Pergamon Press (1976), a description is provided for basic Doppler velocimeters and their usefulness in medical and industrial fields. Atkinson notes that in useful Doppler systems, as opposed to theoretical systems, that the received signal will exhibit a range of Doppler difference spectrum rather than a single frequency predicted by a perfect system. This range of spectrum will be exhibited by a "hump" or bell-shaped curve in the frequency domain. The cause may be the propagation of a finite width beam as opposed to an arbitrarily narrow pulse or may be caused by a finite length of pulse in a pulsed system as opposed to an infinitely short pulse. Atkinson also discloses that the reflection (backscatter) from blood will be amplitude modulated due to differences in time of the volume of red blood corpuscles.
An article by Newhouse et al, "The Effects of Geometric Spectrum Broadening On Ultrasonic Doppler Flow Measurement Systems", 29th ACEMB Proceedings, p. 140 (1976) discusses that spectrum broadening in ultrasonic Doppler flow systems is due to geometric broadening.
An article by Lunt, M. J., "Accuracy and Limitations of the Ultrasonic Doppler Blood Velocimeter and Zero Crossing Detector", Ultrasound in Medicine and Biology, Volume 2, pp. 1-10 (1975), discusses the use of zero crossing detectors in ultrasonic Doppler blood flow measurement.
An article by Brody, "Theoretical Analysis of the CW Doppler Ultrasonic Flowmeter", IEEE Transactions on Biomedical Engineering, Volume BME-2, No. 3, pp. 183-192 (1974) discusses the theoretical basis for ultrasonic continuous wave Doppler blood flowmeters.
A portion of a Chapter from Sears et al, College Physics, Fourth Edition, pp. 366-367, Addison-Wesley Publishing Company (1974) describes the basic Doppler effect as related to acoustic phenomenon.
A book by Atkinson & Woodcock, Doppler Ultrasound and its Use in Clinical Measurement, Chapters 1 and 3, Academic Press (1982) provides an introduction into Doppler sound wave theory and its reaction to the measurement of blood and exemplary systems for the processing and analysis of Doppler shift signals. This books provides a good discussion of the conversion of the Doppler from the time domain to the frequency domain.
An article by Murphy and Rolfe, "Application of the TMS320 Signal Processor for the Real-Time Processing of the Doppler Ultrasound Signal", IEEE/Eighth Annual Conference of the Engineering in Medicine and Biology Society, pp. 1175-1178 (1986) describes techniques to achieve real-time processing of Doppler ultrasound signals applied to the measurement of blood flow. Murphy et al uses Fast Fourier Transform (FFT) techniques to convert from the time domain to the frequency domain and to digitally obtain the average frequency which corresponds to the blood flow measured.